Photoionization (PI) involves directing light of desired wavelengths to an unknown gas sample to induce ionization. PI may be used to facilitate examination of the composition of the unknown gas sample via photochemistry applications such as soft ionization and photo-fragmentation. For example, light from the vacuum ultraviolet (VUV) region of the electromagnetic spectrum is particularly useful in PI applications applications because the energies of VUV photons (generally 6 eV-124 eV) correspond to electronic excitation and ionization energies of most chemical species.
Electron impact ionization (EI) involves directing electrons having a desired kinetic energy at an unknown gas sample to induce ionization and fragmentation of molecules of the sample gas.
Fragmentation by EI is often referred to as “hard” ionization. By contrast, a PI source functions as a “soft” ionization source because sample molecules are less fragmented than when EI sources are used. Less fragmentation of molecules provided by a PI source can produce molecular ion signals to a greater extent than known EI sources.
While the comparatively lesser extent of fragmentation provided by PI sources is useful in certain applications such as the identification of unknown compounds, the fragmentation pattern produced by an EI source provides information that is often useful, but that may not be realized by photo-fragmentation with a PI source.
What is needed, therefore, are ionization devices and methods of use that allows for selective ionization by EI and PI.